Composite materials offer potential design improvements in gas turbine engines. For example, in recent years composite materials have been replacing metals in gas turbine engine fan blades because of their high strength and low weight. Most metal gas turbine engine fan blades are titanium. The ductility of titanium fan blades enables the fan to ingest a bird and remain operable or be safely shut down. The same requirements are present for composite fan blades.
A composite airfoil has a root, which connects to the fan mechanism, and a tip opposite the root. A composite airfoil for a turbine engine fan blade is typically designed with a divergent root portion known as a dovetail root. The thickness of the airfoil greatly changes over a short length at the dovetail root. The dovetail root enables the airfoil to withstand typical operational loads from rotation and bending and loads from foreign object strikes.
The composite airfoil can have a sandwich construction with a three-dimensional woven core at the center and two-dimensional filament reinforced plies or laminations on either side. To form the composite airfoil, individual two-dimensional laminations are cut and stacked in a mold with the woven core. The woven core extends from the root to the tip of the airfoil and the plies are stacked on either side of the woven core to form the desired exterior surface profile. The mold is injected with a resin using a resin transfer molding process and cured.
Historically, the woven core has had an approximately uniform thickness over its length, and the dovetail root has been formed by laying-up plies at the dovetail root to achieve the desired thickness. Thus, there are more plies located at the root than are located at the tip. In one example, 100-200 plies are stacked at the root to achieve the desired thickness while less than 20 plies are present at the tip.
A ply drop is located at the end of each ply. Previous designs resulted in a large number of ply drops along the length of the blade due to the large difference in the number of plies at the root compared to the tip. Ply drops are undesirable because they can be initiation sites for damage and cracks. The weakest region for laminated composites is the interlaminar region between the laminates. High interlaminar shear stresses, such as from operational loads and foreign object strikes, in a laminated composite can cause delamination that compromises the structural integrity of the structure.